Wireless user equipment for use in reducing cross cell interference

ABSTRACT

A method for reducing cross cell interference in a wireless time division duplex communication system using code division multiple access, the system having at least one user equipment (UE) and a base station (BS) is disclosed. The method begins by measuring an interference level of each timeslot at the BS. A timeslot is eliminated for additional uplink communication if the measured interference level exceeds a first threshold. UEs in nearby cells that are large interferers are identified and their downlink timeslot usage is gathered. A timeslot is eliminated for uplink communication for a large interferer UE that uses the timeslot for downlink communication.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/099,325, filed Apr. 5, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/427,174, filed May 1, 2003, now U.S. Pat. No.6,882,849, which is a continuation of application Ser. No. 10/003,487,filed Nov. 1, 2001, now U.S. Pat. No. 6,591,109, which claims thebenefit of U.S. Provisional Application No. 60/313,336 filed Aug. 17,2001, which are incorporated by reference as if fully set forth herein.

FIELD OF THE INVENTION

The invention generally relates to wireless time division duplex (TDD)communication systems using code division multiple access (CDMA). Inparticular, the invention relates to reducing cross cell user equipmentinterference in such systems.

BACKGROUND

FIG. 1 is an illustration of a wireless TDD/CDMA communication system10. The communication system 10 has base stations 12 ₁ to 12 _(n) (12)which communicate with user equipments (UEs) 14 ₁ to 14 _(n) (14). Eachbase station 12 has an associated operational area or cell. The basestation communicates with UEs 14 in its cell.

In CDMA communication systems, multiple communications are sent over thesame frequency spectrum. These communications are distinguished by theirchannelization codes. To more efficiently use the frequency spectrum,TDD/CDMA communication systems use repeating frames divided intotimeslots, such as fifteen timeslots, for communication. In TDD, eachcell's timeslots are used solely for either the uplink or downlink at atime. A communication sent in such a system has one or multipleassociated code(s) or timeslot(s) assigned to it. The use of one code inone timeslot with spreading factor of sixteen is referred to as aresource unit.

Cross cell interference is a problem in such systems as illustrated inFIG. 2. If two different cell's UEs 14 are close to each other, theiruplink transmissions interfere with the other UE's downlinktransmissions in the same timeslot. As shown in FIG. 2, UE 14 ₁ uplinktransmission U1 interferes with UE 14 ₂ downlink transmission D2.Likewise, UE 14 ₂ uplink transmission U2 interferes with UE 14 ₁downlink transmission D1. Although the effective isotropic radiant power(EIRP) of UEs 14 is much less that base stations 12, the close proximityof the UEs 14 results in the unacceptable interference. This problem isexacerbated when adding new users or user services. Although a cell'sbase station and UE 14 may make timeslot interference measurements, suchas interference signal code power (ISCP), to assure its newtransmissions will not see unacceptable interference, other cells' usersmay end up experiencing unacceptable interference due to the newtransmission. As a result, existing calls may be dropped or unacceptablequality of service (QOS) may occur.

Accordingly, it is desirable to reduce cross cell interference.

SUMMARY

A method for reducing cross cell interference in a wireless timedivision duplex communication system using code division multipleaccess, the system having at least one user equipment (UE) and a basestation (BS) is disclosed. The method begins by measuring aninterference level of each timeslot at the BS. A timeslot is eliminatedfor additional uplink communication if the measured interference levelexceeds a first threshold. An interference level of each timeslot ismeasured at the UE, and the timeslot is eliminated for downlinkcommunication for the UE if the measured interference level exceeds asecond threshold. UEs in nearby cells that are large interferers areidentified and their downlink timeslot usage is gathered. A timeslot iseliminated for uplink communication for a large interferer UE that usesthe timeslot for downlink communication.

A system for reducing cross cell interference in a wireless timedivision duplex communication system using code division multiple accessincludes a user equipment (UE), a base station (BS), and a Node B. TheUE includes an interference measurement device for measuringinterference in a timeslot, a transmitter, and a receiver. The BSincludes an interference measurement device for measuring interferencein a timeslot, a transmitter, and a receiver. The Node B includes aresource allocation device configured to receive interferencemeasurement values from the UE and the BS; eliminate timeslots forcommunication where the measured interference exceeds a first threshold;identify UEs in nearby cells that are large interferers; gather downlinktimeslot usage for large interferer UEs; and eliminate a timeslot foruplink communication for a large interferer UE that uses the timeslotfor downlink communication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a wireless TDD/CDMA communication system.

FIG. 2 is an illustration of cross interference between UEs.

FIG. 3 is a flow chart for UE cross cell interference reduction.

FIG. 4 is a flow chart for determining potentially interfered UEs.

FIG. 5 is an illustration of neighboring cell UE usage.

FIG. 6 is an illustration of large interfering UE timeslot usage.

FIG. 7 is an illustration of available UE timeslots.

FIG. 8 is a simplified UE cross cell interference reduction system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the UE cross cell interference reduction is explained in thecontext of unsectorized cells, the approach is extendable to any UEoperating area division, such as sectors of a cell. In such anextension, each operating area, such as a sector, is treated as aseparate cell in the analysis.

FIG. 3 is a flow chart for UE cross cell interference reduction. Foreach cell, the cell's base station 12 measures the interference level ineach timeslot, such as by using ISCP, step 22. The measured interferencein each timeslot is compared to a threshold. If the measuredinterference in a timeslot exceeds the threshold, that timeslot iseliminated as a timeslot for any additional uplink communications inthat cell, step 23. The threshold level is typically set by the systemoperator.

Each UE 14 measures the interference level in each timeslot, such as byISCP, step 24. To determine available downlink timeslots for aparticular UE 14, the measured interference in each timeslot is comparedto a threshold. The threshold level is typically set by the systemoperator. If the measured interference exceeds the threshold, thattimeslot is eliminated for the downlink for that particular UE 14, step25.

Another concern is whether a particular UE's new uplink transmissionswill interfere with another cell's UE downlink transmission. In TDD, UEs14 in the same cell do not transmit on uplink and downlink in the sametimeslot. Since the transmissions are new, other cells' UEs 14 cannotmeasure the resulting interference levels until the new transmissionsbegin. These new transmissions may result in a drop of a user orunacceptable QOS for existing users.

Determining other nearby cells' UEs 14 which may interfere with aparticular UE 14 is per the flow chart of FIG. 4. Each neighboringcell's UE uplink timeslot usage is gathered, step 29. This usage istypically stored at the radio network controller (RNC) 42 and/or at theNode-B 46 (see FIG. 8). Only the UE usage of nearby cells or,alternately, only adjacent cells are used. Further cells' UEs 14 are toofar away to suffer interference from the particular UE 14. An example ofnearby UE uplink usage is shown in FIG. 5. Each UE 14 is represented bya different letter, “B” to “L”. The particular UE 14 is an unshownletter “A”.

Using the particular UE's timeslot interference measurements, thetimeslots are categorized into either a large or small interferencecategory, step 30. The small or large interference determination isperformed such as by a threshold test. The threshold is typically set bythe system operator. All nearby cell UEs 14 transmitting uplinkcommunications in timeslots having a small interference are consideredtoo far away to suffer interference from the particular UE's uplinkcommunications, step 31. All the other UEs are considered to bepotentially interfered with by this UE's uplink communications, step 32.

To illustrate using the example of FIG. 5, UE A has nearby UEs B-L.Uplink timeslots are indicated with a “U”. Out of the eight uplinktimeslots (slots S1, S3, S5, S7, S9, S11, S13, S15), three slots havelarge interference (slots S1, S3 and S7) and five have smallinterference (slots S5, S9, S11, S13 and S15). The UEs 14 transmittingin small interference uplink slots are UE C, D, F, G, H, I, J, K and Land in large interference uplink slots are UE B, D, F and H. Although UED and F have an uplink transmission in a large interference cell, theyalso have an uplink transmission in a small interference cell. As aresult, UE D and F are not considered the interfering UEs 14 in thelarge interference timeslots. In this example, UE B and H are determinedto be the interfering UEs.

In this simplified example, there was no ambiguous information. However,ambiguous information may exist. For instance, if UE H also had anuplink transmission in a small interference cell, such as slot S9, theinformation is ambiguous. UE H would be considered both a largeinterferer in slot S7 (being the only uplink user) and a smallinterferer in slot S9. In a conservative implementation, UE H could bedeemed a large interferer. In a more aggressive implementation, UE Hcould be deemed a small interferer. There may be an unaccounted forinterferer or interference source in that timeslot (slot S7).

Another situation where ambiguous information may occur is wheremultiple potential large interferers transmit uplink communications inthe same timeslots. To illustrate, UE H may also transmit in the uplinkin slots S1 and S3. As a result, UE B may or may not be a largeinterferer. UE H may be the only large interferer. In this case, UE B isstill deemed a large interferer to be conservative.

After the large interferer UEs 14 are determined, step 26 (FIG. 3),those UEs' downlink timeslot usage is gathered, such as in FIG. 6, step27. For all the timeslots that the large interferers use for thedownlink, that timeslot is eliminated for the uplink for that UE, asshown by an “X”, step 28. As a result, a table such as in FIG. 7 isproduced. The table indicates which timeslots are available to theparticular UE 14. The available timeslots are blank and thenon-available have an “X”. Timeslots are assigned to the particular UEby selecting from the non-eliminated timeslots.

FIG. 8 illustrates a simplified system implementation for cross cell UEinterference reduction. The RNC 42 has a resource allocation device 44.The resource allocation device 44 allocates the resources, such as codeand timeslot assignments, for the cells. The resource allocation device44 has an associated memory 45 for storing information, such as UE codeand timeslot assignments, interference measurements and UE timeslotavailability lists. Depending on the type of system, the computationalcomponent of cross cell UE interference reduction may be performed bythe RNC resource allocation device 44, the Node-B resource allocationdevice 48 or shared between the two. Typically, performing thecomputation at the Node-B 46 allows for faster updates.

The Node-B 46 communicates with the radio network controller 42. TheNode-B 46 has a resource allocation device 48 and an associated memory49. The resource allocation device 48 allocates resources to thatNode-B's users. The resource allocation device memory 49 storesinformation, such as the Node-B's UE timeslot and code assignments,interference measurements and UE timeslot availability lists.

The Node-B 46 typically communicates with a group of base stations 12.The base station 12 has a channel code and timeslot controller 54. Thechannel code and timeslot controller 54 controls the timeslots andchannel codes assigned to user communications as directed by the Node-B46 and RNC 42. A modulation and spreading device 56 processes data to betransmitted to the users. The data is processed to be time multiplexedwith a channel code as directed by the channel and timeslot controller54. A transmitter 52 formats the processed data for transfer over theradio interface 78. The resulting signal passes through an isolator orswitch 58 and is radiated by antenna or antenna array 60.

Signals are received by the base station 12 using the antenna or antennaarray 60. The received signals pass through the isolator or switch 58 toa receiver 50. The receiver 50 processes the received signals withchannel codes in the timeslots directed by the channel code and thetimeslot controller 54 to recover the received user data. The basestation 12 also has an interference measurement device 74. Theinterference measurement device 74 measures the timeslot interferencelevels.

The UE 14 receives signals over the radio interface 78 using its antennaor antennal array 62. The received signals pass though an isolator orswitch 64 to a receiver 68 to recover the received data for the user asdirected by the channel code and timeslot controller 70. The channelcode and timeslot controller 70 sends the channel code and timeslotinformation to the receiver 68 and UE modulation and spreading device72. The controller 70 also retrieves the code and timeslot assignmentssignaled by the base station 12.

A UE interference measurement device 76 measures the interference levelsin the timeslots. The modulation and spreading device 72 processes userdata with the channel codes and timeslots as directed by the UEcontroller 70. The processed data is formatted for transmission over theair interface 78 by the transmitter 66. The resulting signal passesthrough the isolator or switch 64 and is radiated by the antenna orantenna array 62.

1. A method for reducing cross cell interference in wirelesscommunications, comprising: measuring an interference level of eachtimeslot at a base station (BS); eliminating a timeslot for additionaluplink communication if the measured interference level exceeds a firstthreshold; identifying user equipments (UEs) in cells near a particularUE that are large interferers; gathering downlink timeslot usage forlarge interferer UEs; and eliminating a timeslot for uplinkcommunication for a large interferer UE that uses the timeslot fordownlink communication.
 2. The method according to claim 1, wherein themeasuring includes measuring the interference signal code power of atimeslot.
 3. The method according to claim 1, wherein the identifyingincludes: gathering uplink timeslot usage for nearby UEs; categorizingeach timeslot based on the measured timeslot interference, wherein atimeslot is a small interference timeslot if the measured interferenceis below a second threshold and a timeslot is a large interferencetimeslot if the measured interference exceeds the second threshold;identifying a UE as non-interfering if the BS receives from the UE inthe uplink in a small interference timeslot; and identifying a UE as alarge interferer UE if the BS does not receive in the uplink in a smallinterference timeslot.